“Tracey met a violent and horrific death at the hands of her killers,” said Det Insp Gary McIntyre.

“She was so brave and told detectives as much as she could before she died.

“I believe there are people out there who may have witnessed what happened either in Birmingham, where Tracey was taken, or Cheshire, where she was killed.

“I would specifically appeal to them to come forward.”

Tracey had moved from Birmingham to Rochdale the month before she died but returned to collect a benefits book from her old address.

Police believe she initially intended to return the same day but instead ended up staying overnight at her sister-in-law’s and visited her former home the next morning.

Shortly before noon on December 23 she answered a knock at the door and was confronted by two black men, who were believed to have spoken with a Jamaican accent.

Police believe the dinner lady was abducted and bundled into the back of a yellow Mark II Ford Escort parked outside.

She suffered 95 per cent burns and was found in a churchyard in the village of Eaton, between Macclesfield and Congleton, around five hours later.

A member of the public came to her aid but Tracey could not be saved and no-one has ever been convicted in connection with her death.

Speaking of the tragedy in 2011, her sister Sharon Howarth said: “It was an horrendous time, just awful.

“There was no Christmas that year and Christmas isn’t the same any more.

“Every year the memories are back.

“It still hurts and I still miss her.

“She was lovely, chatty, really fun. She lived for her children.”

Det Insp McIntyre said detectives had followed up “numerous lines of inquiry” in the past 20 years but had drawn a blank.

“By refreshing people′s memories with a renewed appeal it is hoped that further information could be gained, which could finally help to bring Tracey′s killers to justice,” he said.

“I appreciate it is a long time ago, but I am sure that if you witnessed something you would still remember this now. I would urge anyone with any information to come forward and help us with our investigation.”

The case featured on Crimewatch in 2010 and 2012 after advances in forensic technology gave detectives a DNA profile of the killer.

They followed up more than 100 pieces of information after the 2010 broadcast but none provided a breakthrough.

Anyone with any information should contact the Operation Eaton investigation team on 101 or Crimestoppers, in confidence, on 0800 555 111.